It is known to protect telephone equipment from abnormally high voltages that may be found occasionally on the tip and ring conductors of a subscriber line. These abnormally high voltages are caused by lightning strikes or power crosses, e.g. a conductor falls across a power line and the telephone line to be protected.
There has recently been a development in the telecommunications industry to transmit high density or digital information on existing telephone lines leading to homes. Although fiber optic strands are now widely used in new residential construction, older construction used copper wire. By far the majority of telephone wire leading to residences is copper.
A technological struggle is going on, and will continue, between three industrial groups: telephone companies who want to provide entertainment and other services along with conventional telephone service, cable TV companies who want to provide conventional telephone service and other information related services along with their existing entertainment services and entrepreneurs who wish to provide telephone, entertainment and other services via satellite transmission.
Telephone companies desire to use existing copper wires leading to residences to transmit the high density information needed to deliver entertainment and other services. It makes much more sense to modify existing telephone equipment rather than pay the tremendous cost, and suffer the delay, of rewiring most existing neighborhoods. One problem that has occurred involves the surge protection system currently used to prevent damage to circuit board components in central offices. A substantially identical surge protection system is envisioned for high density applications to prevent damage to circuit board components at central offices, adjacent residences and on repeaters between central offices and residences.
Conventional surge protection systems have two or more stages of surge protection: a primary stage provided by a gas discharge device and a secondary stage incorporating conventional surge protectors which are the predecessor of this invention and which provide a 20-100 ohm resistor adjacent a meltable fuse. For years, applicant's assignee and others have supplied secondary stage surge protectors comprising a housing, a 20-100 ohm wound wire resistor, a fuse and a potting cement filling the housing and encapsulating the resistor and fuse.
Considerable effort has been spent by telecommunications companies to determine the optimum or desirable resistance of the secondary surge protector resistor. All evidence points to values less than 10 ohms and the presently preferred value is 5.6 ohms.
At the outset, prototype surge protectors were made using conventional technology except the resistor was modified to be 5.6 ohms rather than the more conventional 20-100 ohms and then tested to determine if they were satisfactory. They were not.
When the voltage potential across the resistor is sufficient to create a current surge, the resistor either heats up rapidly and breaks at high current or heats up slowly and melts the fuse at a lower current. Conventional surge protectors having 5.6 ohm resistors failed the appropriate test because they essentially exploded during some test of the procedure. This produces a fire hazard, potentially damages an expensive circuit board from fire or fragmentation and is manifestly unacceptable.
The procedure used to determine whether a particular surge protector is adequate is known as Bell Spec TR-1089. Bell Spec TR-1089, incorporated herein by reference, is a complicated test procedure, about 30 pages long, designed to determine whether a particular surge protector is adequate to handle a variety of events that can damage telephone equipment. There are two events the surge protector must handle: lightning strikes and power crosses, e.g. a conductor falls across a live power line and a telephone line. To satisfy the lightning strike requirement, the surge protector must accept, and still work after, the tests shown in Table I:
TABLE I ______________________________________ Lightning ______________________________________ 1000 volts - 10/1000 usec pulse - 25 hits each polarity 2500 volts - 2/10 usec pulse - 10 bits each polarity. ______________________________________
The power cross requirements are considerably more complicated and are summarized in Table II:
TABLE II ______________________________________ Power Cross ______________________________________ 1. First Level - line simulation fuse (MDQ 1-6/10 amps) in series with test resistor - fuse should not open during tests 50 volts - 150 ohms series resistance - 15 minutes - stay in spec 100 volts - 600 ohms series resistance - 15 minutes - stay in spec 200, 400, 600 volts - 600 ohms series resistance - 60 applications - stay in spec 100 volts - 1000 ohms series resistance - 60 applications - stay in spec 2. Second Level 120 volts - 25 amps - 15 minutes - open circuit or stay in spec 277 volts - 25 amps - 15 minutes - open circuit or stay in spec 600 volts - 60 amps - 5 seconds - open circuit 600 volts - 40 amps - 5 seconds - open circuit 600 volts - 7 amps - 5 seconds - open circuit 100 to 600 volts - 2.2 amps at 600 volts - 15 minutes - open circuit or stay in spec ______________________________________